Oxidative and color stability of rosin resins, particularly rosin esters, are very important to customers who use these resins in products such as adhesives, lacquers, and varnishes. While it is fairly easy to improve the oxidative stability of rosin resins by addition of a hindered phenol antioxidant, improvement in color stability is much more difficult to achieve.
Rosin is composed mainly of a mixture of diterpene acids, known as resin acids, with a smaller amount of other acidic and neutral bodies present. Rosin characteristically contains color bodies or coloring matter visible to the naked eye and certain color bodies that are not normally visible to the naked eye, usually referred to as latent color bodies.
The visible coloring matter in a tall oil, wood and gum rosin determines its grade and hence its value. The darker the rosin, the lower its grade and value. In the case of tall oil rosin, the latent color bodies it contains do not color the rosin until it is treated with pentaerythritol or other polyols to form an ester, or heated in the presence of oxygen, such as in the manufacture of hot melt adhesives. This discoloration renders the polyol esters produced from tall oil undesirable where a light colored product that retains its color under normal conditions of use is required.
Many methods have been described for improving the color and color stability of rosin resins. For example, U.S. Pat. No. 4,657,706 discloses heating tall oil rosin in the presence of a carboxylic acid and a Lewis acid catalyst, distilling the reaction mixture and esterifying the distilled tall oil rosin with a polyol. U.S. Pat. No. 4,657,703 discloses heating tall oil rosin in the presence of a Lewis acid catalyst, distilling the reaction mixture, and esterifying the tall oil rosin distillate with a polyol in the presence of a phenol sulfide. Japanese examined patent application JP 72-000615 describes a method for producing a rosin resin of pale color by treating the rosin with a sulphoxylic acid salt-formaldehyde adduct at 200.degree.-260.degree. C. in an inert gas stream. U.S. Pat. No. 3,780,012 describes a process for improving the color of tall oil rosin pentaerythritol ester by pretreating a tall oil rosin with paraformaldehyde at a temperature of 140.degree.-180.degree. C. for one to three hours.
Although sodium hydroxymethane sulfonate, also known as sodium bisulfite formaldehyde, has been used as a preservative in cosmetic compositions (see P. Alexander, Cosmetics and Toiletries, Vol. 102, pp. 89-92, December, 1987), its use in rosin resins has not been disclosed.